Dopant diffusion is a standard process that is used when fabricating some semiconductor-based devices. One important application for this process is the creation of a buried “p-n junction.” A p-n junction is formed when p-type and n-type materials are placed in contact with each other. The junction between the two types of materials behaves very differently than either type of material alone. In particular, current will flow readily in one direction but not in the other, creating the basic diode. This behavior arises from the nature of the charge transport process in the two types of materials.
To create a buried p-n junction via dopant diffusion, a diffusion mask is first formed on a wafer, such as an indium phosphide wafer, on which epitaxially grown layers have been deposited. Typically, the grown layers are characterized by a background concentration of either n-type or p-type dopant, resulting in n-type material or a p-type material, respectively. The mask includes “windows” that are open to the underlying semiconductor at specific regions. The specific arrangement of the windows in the mask is based on the layout and geometry of the devices being formed. Dopant atoms of the type opposite to those in the epitaxially grown layers (i.e., “n” or “p”) are delivered to the diffusion mask. The dopant atoms pass through the windows and diffuse to a certain depth in the semiconductor to form p-n heterojunctions. The diffusion process is controlled via parameters such as temperature and dopant partial pressure. Typically, elevated temperature (c.a. 500° C.) is required to achieve reasonable diffusion times.
Essentially the same process can be used to create a homojunction wherein both sides of the junction have the same dopant type, but with very different concentrations thereof.
The buried p-n junction is an important element of many optoelectronic devices, such as the avalanche photodiode (“APD”). An APD is a photodiode that can generate a relatively large electrical current signal in response to receiving a relatively low-power optical signal. Some versions of APDs include an absorption layer that absorbs the energy from light to create free charge carriers and another layer that contains a multiplication region in which the free charge carriers multiply to create a detectable electrical current. APDs with such separate absorption and multiplication regions are referred to as “SAM” APDs.
In operation, an APD is “biased” by applying a voltage across it to create a high electric field. Free electrical carriers generated in the absorption layer are injected into the multiplication region. In the multiplication region, the free carriers are accelerated to a velocity that enables them to create more free carriers through a process called “impact ionization.” The resultant additional free carriers are also accelerated by the electric field and create even more free carriers, and so on. This process is referred to as avalanche multiplication, and is responsible for the high responsivity of an APD.
FIG. 1 depicts a cross-sectional view of a conventional SAM APD receiving an optical beam. APD 100 comprises substrate 102, absorption layer 104, charge control layer 106, and cap layer 108. The various layers 104, 106, and 108 are grown on substrate 102. APD 100 is a top-illuminated device; that is, light is received at the uppermost layer of the device. With slight design alterations, such as the location of an anti-reflection layer (not depicted in FIG. 1), an APD can be back illuminated, wherein light is received at the bottommost layer of the APD.
Absorption layer 104 absorbs the optical energy of optical beam 114, as contained within mode-field diameter 116, and generates electrical carriers. The absorption layer can be an intrinsic layer of indium gallium arsenide. Charge control layer 106 permits a low electric field to be maintained in absorption layer 104, while supporting a high electric field in cap layer 108. Charge control layer 106 can be a moderately n-doped layer of indium phosphide.
Cap layer 108 includes device region 110 and p-n junction 112. Cap layer 108 can be a lightly n-doped layer of indium phosphide.
Device region 110 is formed by diffusing a p-type dopant into cap layer 108 to form p-n junction 112. The lateral extent of the p-n junction defines the lateral extent of device region 110. As indicated via the smooth shape of p-n junction 112, it was formed via a single dopant diffusion operation.
As depicted in FIG. 2, which shows further detail of cap layer 108 of the APD depicted in FIG. 1, device region 110 includes active region 220 and edge region 226. Active region 220 is the central portion of device region 110 wherein p-n junction 112 is at a uniform depth (i.e., where p-n junction 112 is a plane junction). In some embodiments, such as the one depicted, width 222 of active region 220 is substantially equal to mode-field diameter 116 of optical beam 114. In some other embodiments, width 222 is made larger than mode-field diameter 116 in order to facilitate optical coupling to optical beam 114.
Avalanche multiplication region 224 is the un-doped portion of active region 220 beneath p-n junction 112. This is a high-field multiplication region in which avalanche multiplication occurs. The avalanche multiplication region 224 has a thickness Tm, which is substantially constant across active region 220. Depending upon device design considerations, thickness Tm is a value in the range from 2% to 50% of the thickness of cap layer 108.
Edge region 226, which has width 228, is the outer or peripheral region of device region 110. The edge region is formed by the lateral diffusion of the dopant used to form p-n junction 112. Within edge region 226, p-n junction 112 is non-planar and varies across edge region 226.
FIG. 3 depicts a cross-sectional view of cap layer 108 showing equipotential lines. The junction depth of p-n junction 112 in the active region is approximately equal to diffusion radius rj. In the prior-art APD depicted in FIGS. 1-3, width 222 of the active region 220 is much larger than width 228 of the edge-region and is also much larger than diffusion radius rj.
Two important parameters for an APD are the uniformity of the gain and breakdown voltage across device region 110. The breakdown voltage is the voltage at which the p-n junction is sufficiently reverse-biased to conduct a large current arising from a self-sustaining avalanche process, even in the absence of optical power.
Gain and breakdown voltage are functions of the thickness of the un-doped portion (i.e., the region of cap layer below p-n junction 112) of the device region 110. As previously noted, the active region is characterized by a uniform, planar junction profile, which results in uniform avalanche gain and breakdown voltage, as denoted by the uniform space between equipotential lines 336 in this region. The edge region, however, has a non-uniform, curved junction profile, which gives rise to the well-known “junction-curvature” effect. The junction curvature effect leads to higher electric field intensity and lower breakdown voltage relative to the active region, as denoted by the crowding of equipotential field lines 336 in regions 338. This phenomenon is commonly referred to as “edge breakdown.” For practical SAM-APDs, the breakdown-voltage should be sufficiently uniform across the entire device region to ensure that the resulting uniformity of the multiplication gain is within 10%, and preferably within 1%.
A number of approaches for limiting edge breakdown are known in the prior art. These include: 1) adding guard rings outside the junction area to control the doping density at the junction edges; 2) adding a shaped charge control layer underneath the cap layer to enhance the electric field in the active region; and 3) forming a multi-tiered doping profile (i.e., two or more separate dopant diffusions) to reduce the curvature, and therefore the induced local electric field, of the junction profile at the edge of the device region. Common to all these approaches is an enlarged device region and/or more complicated device fabrication, which can lead to lower device yield, higher device cost, and lower device reliability.
A fourth approach to limiting edge breakdown is disclosed in Chi et al., “Planar Avalanche Photodiode with a Low-Doped, Reduced Curvature Junction,” Appl. Phys. Lett. 50 (17), 27 Apr. 1987. The authors disclosed that a dish-shaped p-n junction can be used to provide uniform gain without edge breakdown. The dish-shaped junction is formed by etching a dish-shaped recess into an implant mask layer, which is reproduced in the cap layer of the APD. The dish-shaped recess was formed using a photomask that produced a spatial variation of light intensity.
Of late, interest has arisen in the use of APDs for detection of single photons in applications such as cryptography. For single-photon-detection applications, the APD is electrically biased beyond its electrical breakdown voltage, in the so-called “Geiger mode.” These devices are often referred to as Geiger-mode (Gm) APDs. A GmAPD that is biased above breakdown can give rise to an easily detectable pulse of electrical current in response to the absorption of even a single photon.
An important performance metric for an APD that is used for single-photon detection application is Noise Equivalent Power (NEP). NEP is a function of the amount of erroneous signals (referred to as the dark count rate) and the optical detection efficiency. A photodiode with low NEP will contribute few false counts while still detecting most or all of the received photons. A low NEP can be achieved by 1) high detection efficiency and/or 2) low dark count rate.
In U.S. Pat. No. 7,378,689, incorporated herein by reference, the present inventor disclosed an APD that provides high optical coupling efficiency and low dark count rate, such as is particularly useful for single-photon detection applications. This patent discloses the dark count rate can be decreased by reducing the volume of the device region. Note that this approach is contra-indicated from the perspective of limiting edge breakdown, at least via traditional approaches 1-3 mentioned above. In some embodiments, the total volume of the device region is reduced by decreasing the width of the edge region. This patent also teaches that the curvature of the junction profile in the edge region (and therefore the gain and breakdown voltage in the edge region) is affected by the ratio of the width of the edge region to: (1) the width of the active region; (2) to the diffusion radius of the dopants in the edge region; and (3) to cap layer thickness. The patent specifies certain ranges for these ratios, providing an APD that operates with low NEP while maintaining a uniform gain profile or uniform breakdown voltage across the device region.
It will be appreciated that APD breakdown voltage is highly dependent upon APD layer structure and the properties of the semiconductor layers from which it is formed. These factors have historically been extremely difficult to control from wafer to wafer, fabrication run to fabrication run, and even across a single wafer within a run. Local variations in process parameters, such as temperature and/or gas flow, can lead to significant variations in the breakdown voltage across a wafer of APD structures.
While it is possible to adjust for a variation in expected breakdown voltage for a single APD, it can be quite complex and costly to measure and compensate for individual breakdown voltages within an array of such devices. For applications in which a plurality of APDs is required, such as in imaging sensors, position sensors, etc., the impact of breakdown voltage variation is, therefore, particularly vexing and problematic. Such variation necessitates costly inspection methods, complex control circuitry, increased cost, and often a degradation of the performance of the APD array.
As a consequence, there remains a need for improved methods for fabricating APDs.